But little is known about how quickly an avian virus can develop the ability to pass easily from person to person.

That's a huge percentage of deaths, but the virus has not yet developed the ability to spread easily from human to human.

The virus developed the three expected resistance mutations and became inactive.

The likelihood that the virus will develop resistance to a certain class of medicine.

The virus would "develop" the ability to move from human to human.

But neither does the virus develop resistance to the drugs in people who follow the strict schedule of medicine.

The scientists used calculations to estimate the date when the virus first developed.

A crucial question is whether the virus could develop resistance to the drugs.

The virus may also develop a resistance to Tamiflu, although that has not been documented yet.

About 80% of those exposed to the virus develop a chronic infection.